I'm also guessing that the timer means they only have a short amount of time left before the full blown alien invasion we all thought was going to happen. But I don't think the manga's going to end any time soon. Personally, I've always thought that Phase 1 was the introduction of the story, Phase 2 will be the start of the alien invasion and maybe there will be a Phase 3 with all the Gantz teams uniting to free the planet.

haha, if that's true then nishi is more veteran than we thought, who knows that nishi know some secret? Maybe it has happened before, and that's why nishi know that there's others people in different country fighting in gantz room too